For certain applications, such as continuous, proportional controllers, it is desirable to operate a multi-pole, multi-phase synchronous machine, such as a synchronous motor, at variable speed of variable torque angle. It is important to control torque angle in applications wherein efficiency and power factor are maximized. (In the present specification and claims, the term "torque angle" is the electrical angle between the voltage applied to a terminal of the machine, referenced to neutral, and a voltage generated internally by the machine. If the machine is a multi-phase, wye connected motor, the torque angle is the angle between the applied motor terminal voltage and neutral and the internally generated, back EMF of the motor caused by the field winding.) Speed control of such a synchronous motor is important in applications relating to electrically powered vehicles, variable speed compressors and fans.
Numerous systems have been developed to control the speed or torque angle of a synchronous motor. Recently, these systems have employed pulse techniques and frequency locked loops wherein a voltage controlled oscillator is responsive to pulses representing the angular displacement of a synchronous motor shaft. These systems, however, have frequently encountered problems in maintaining synchronism under transient conditions, caused either by a change in load or a change in speed or torque angle set point. The transient conditions may cause the machine to "slip a pole". Pole slipping is a phenomenon wherein the transient causes a displacement of the motor shaft by an angle commensurate with the angular displacement of one pole of the motor. If a pole slip does occur, the prior art circuits have generally not included any compensating provision. Instead, the motor shaft has generally remained out of synchronism.
It is, accordingly, an object of the present invention to provide a new and improved control system for a multi-phase synchronous machine.
Another object of the invention is to provide a new and improved system for controlling the speed of a multi-phase synchronous machine.
A further object of the invention is to provide a new and improved system for controlling torque angle of a multi-phase synchronous machine.
A further object of the invention is to provide a new and improved circuit for maintaining a multi-phase synchronous motor in synchronism with a power source under transient conditions.
An additional object of the invention is to provide a new and improved multi-phase synchronous machine system for deriving an output indicative of machine torque angle.
An additional object of the invention is to provide a new and improved pulse-type system for a multi-phase synchronous machine adapted to be driven at a variable output speed or to have a variable torque angle wherein synchronism of the machine is maintained under transient conditions.